Undesirable signals arise during routine eddy current inspection due to variations in separation distance between eddy current sense elements and the underlying inspection surface of a conductive part undergoing inspection. The term "lift off" is a term in the art that refers to the separation distance between sense element and inspection surface. Lift off signals are non-uniform among sense elements of an array whenever the array cannot maintain substantially the same separation between each element and the underlying inspection surface. Lift off contributions are capable of corrupting actual sense element measurement signals, thereby distorting or concealing signals otherwise due to near surface flaws and cracks. Thus, near surface flaw detection is impaired by lift off effects caused by unequal element to surface separation.
Typically, lift off signals are either suppressed or compensated for in a computational manner using signal processing techniques. The compensation method of U.S. Pat. No. 4,661,777 by Tornblom describes typical electronic signal processing on signals of multiple frequencies as a technique of compensation for aberrant lift off signals. U.S. Patent Nos. 4,808,927 by Cecco et al and 4,608,534 by Cecco et al utilize signal phase segregation rather than signal compensation to discriminate aberrant signals due to structural variations from actual measurement signals. Alternatively, a sense element itself can be optimized to be operated in such a way as to suppress or compensate for lift off effects by utilizing sufficiently dissimilar frequencies to generate different current penetration depths. A sense element design is selected to cooperate with the method of suppression and/or compensation providing an interdependent system like that described in U.S. Pat. No. 4,703,265 by Tornblom wherein such a sense element design is described. The sense element therein is designed in such a way that a complete or partial suppression of undesired lift off signal effect is achieved. This is accomplished utilizing a difference in depth of penetration for current induced in the conductive part at different carrier frequencies. Compensation or suppression by this sort of sense element design involves automatically adapting the use of dual excitation frequencies to optimize sense element response to inspection surface structure, thereby adjusting signal to noise ratio to a given type, shape and/or position of sense element. The effects of the difference between depth of current penetration into the inspection part at different frequencies can then be compensated for. A common multi-frequency inspection technique consists of recording data from at least two judiciously chosen, simultaneous frequencies then performing simultaneous frequency mixing in order to blank an unwanted signal that may be due to lift off.
Eddy current arrays have been used to speed up inspection scanning for near surface detection of flaws. However, surface irregularities encountered in array scanning the conductive part under inspection provide array sense element responses which include aberrant signal components contributing to non-uniformity among array sense element signals. This phenomenon jeopardizes the reliable use of arrays in production inspections as their ability to reliably detect near surface defects or flaws in the part is diminished.
It would therefore be desirable to design an eddy current array having means for compensating for aberrant lift off signals experienced by individual array elements. The array should be suitable for use in a production inspection environment.